Research Of Weak Capacitance Sensing Signal Readout Circuit Technology

The capacitive sensor plays an important role in both national defense and civilianfields. With the development of micro electromechanical systems (MEMS), thecapacitance signal of the MEMS micro capacitance sensor is becoming more and moreweak. However, it is difficult to measure a capacitance less than10pF. Thus, the microcapacitance detection technology has become a bottleneck in the development of thecapacitive sensor, and attracts more and more attentions in recent years. Designingstable micro capacitance detection circuits with high accuracy and sensitivity, havebecome an urgent need for our nation.After analyzing the theory and performance of various capacitance measuremethods, the TDC (time to digital converter) technology is adopted in this thesis tomeasure a micro capacitance based PS021chip. This circuit has the complexprogrammable logic device (CPLD) as the main controller, which realizes the timingcontrol of PS021as well as data collection, analysis, processing and display. Both thehardware and software design are modular designed, including the PS021capacitancedetection circuit, the CPLD microcontroller circuit, the JTGA interface circuit, theRS232serial interface circuit, the power supply circuit and the LCD display circuit. Inthis design, the power consumption, low noise, miniaturization and low cost are givenadequate consideration. Meanwhile, to achieve high accuracy, the stray capacitance,parasitic capacitance, and the charge injection of electronic switch are given sufficientconsideration in the process of the main chip selection, measurement mode selectionand PCB design.Capacitance less than10pF and capacitance more than50pF are tested by thiscircuit. The main results are as follows. Firstly, the highest update rate can be optimizedto50KHz and the resolution is improved to8aF(10-18F) at10Hz update rate. Secondly,to measure a capacitance less than10pF, the resolution is improved to23aF and theeffective number of bits (ENOB) is enhanced to18bits. Thirdly, to measure acapacitance more than50pF, the ENOB is improved to22bits with0.01%relativetesting error. Acorrding to the testing results, this circuit achieves good stability with high detection precision and sensitivity.